1. Field of the Invention
This invention relates generally to improvements in magnetically coupled pumps and, more particularly, but not by way of limitations, to improvements in the construction of impellers for such pumps.
2. Description of the Prior Art
Magnetically coupled pumps; that is, pumps having an impeller rotatably mounted within a sealed pump housing and coupled magnetically to an external drive magnetic which is rotated to rotate the impeller, are advantageously used in many pumping applications. For example, the pump may be used to circulate a fluid which is corrosive so that stringent chemical requirements are placed on the materials of which the pump is constructed. If the impeller of the pump is driven by a shaft which passes through a seal in the pump housing, the seal must, in general, meet the same chemical requirements. Since the seal must also have physical properties dictated by its primary function of permitting the shaft to rotate while preventing the escape of fluid from the pump, it can be difficult to find suitable materials for constructing seals. This problem is avoided in magnetically coupled pumps by eliminating the need for such seals.
Problems can also be encountered in magnetically coupled pumps. In many applications, the impellers of such pumps are often subjected to temperature variations in the fluid being pumped. Since the impeller must contain a magnet for coupling to the external drive magnet, differences between the thermal expansion properties of the impeller magnet and other portions of the impeller can result in repeated stressing of the impeller and such repeated stressing can shorten the lifetime of the pump. Moreover, materials which have suitable magnetic properties for construction of the impeller magnet may not have desirable chemical properties with regard to the fluid passing through the pump in a particular application.
These problems can be especially acute when the impeller magnet is formed of a ceramic material. Such materials tend to brittle and they crack when exposed to thermal shock. If the magnet cracks, portions of the magnet may engage the pump housing and jam the pump. One solution to this latter problem is to provide the magnet with a guard member such as that described in U.S. Pat. No. 3,938,914, issued Feb. 17, 1976 to Zimmerman.
It is known that magnets may be formed of magnetic particles imbedded in a plastic matrix. The construction of such magnets has been described in U.S. Pat. No. 3,881,853, issued May 6, 1975 to Loubier.